Network Working Group T. Ylonen
Internet-Draft Helsinki University of Technology
draft-ylonen-ssh-protocol-00.txt 15 November 1995
Expires: 15 May 1996
The SSH (Secure Shell) Remote Login Protocol
Status of This Memo
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The distribution of this memo is unlimited.
Introduction
SSH (Secure Shell) is a program to log into another computer over a
network, to execute commands in a remote machine, and to move files
from one machine to another. It provides strong authentication and
secure communications over insecure networks. Its features include
the following:
o Closes several security holes (e.g., IP, routing, and DNS spoof-
ing). New authentication methods: .rhosts together with RSA
[RSA] based host authentication, and pure RSA authentication.
o All communications are automatically and transparently
encrypted. Encryption is also used to protect integrity.
o X11 connection forwarding provides secure X11 sessions.
o Arbitrary TCP/IP ports can be redirected over the encrypted
channel in both directions.
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o Client RSA-authenticates the server machine in the beginning of
every connection to prevent trojan horses (by routing or DNS
spoofing) and man-in-the-middle attacks, and the server RSA-
authenticates the client machine before accepting .rhosts or
/etc/hosts.equiv authentication (to prevent DNS, routing, or IP
spoofing).
o An authentication agent, running in the user's local workstation
or laptop, can be used to hold the user's RSA authentication
keys.
The goal has been to make the software as easy to use as possible for
ordinary users. The protocol has been designed to be as secure as
possible while making it possible to create implementations that are
easy to use and install. The sample implementation has a number of
convenient features that are not described in this document as they
are not relevant for the protocol.
Overview of the Protocol
The software consists of a server program running on a server
machine, and a client program running on a client machine (plus a few
auxiliary programs). The machines are connected by an insecure IP
[RFC0791] network (that can be monitored, tampered with, and spoofed
by hostile parties).
A connection is always initiated by the client side. The server
listens on a specific port waiting for connections. Many clients may
connect to the same server machine.
The client and the server are connected via a TCP/IP [RFC0793] socket
that is used for bidirectional communication. Other types of tran-
sport can be used but are currently not defined.
When the client connects the server, the server accepts the connec-
tion and responds by sending back its version identification string.
The client parses the server's identification, and sends its own
identification. The purpose of the identification strings is to
validate that the connection was to the correct port, declare the
protocol version number used, and to declare the software version
used on each side (for debugging purposes). The identification
strings are human-readable. If either side fails to understand or
support the other side's version, it closes the connection.
After the protocol identification phase, both sides switch to a
packet based binary protocol. The server starts by sending its host
key (every host has an RSA key used to authenticate the host), server
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key (an RSA key regenerated every hour), and other information to the
client. The client then generates a 256 bit session key, encrypts it
using both RSA keys (see below for details), and sends the encrypted
session key and selected cipher type to the server. Both sides then
turn on encryption using the selected algorithm and key. The server
sends an encrypted confirmation message to the client.
The client then authenticates itself using any of a number of authen-
tication methods. The currently supported authentication methods are
.rhosts or /etc/hosts.equiv authentication (disabled by default), the
same with RSA-based host authentication, RSA authentication, and
password authentication.
After successful authentication, the client makes a number of
requests to prepare for the session. Typical requests include allo-
cating a pseudo tty, starting X11 [X11] or TCP/IP port forwarding,
starting authentication agent forwarding, and executing the shell or
a command.
When a shell or command is executed, the connection enters interac-
tive session mode. In this mode, data is passed in both directions,
new forwarded connections may be opened, etc. The interactive ses-
sion normally terminates when the server sends the exit status of the
program to the client.
The protocol makes several reservations for future extensibility.
First of all, the initial protocol identification messages include
the protocol version number. Second, the first packet by both sides
includes a protocol flags field, which can be used to agree on exten-
sions in a compatible manner. Third, the authentication and session
preparation phases work so that the client sends requests to the
server, and the server responds with success or failure. If the
client sends a request that the server does not support, the server
simply returns failure for it. This permits compatible addition of
new authentication methods and preparation operations. The interac-
tive session phase, on the other hand, works asynchronously and does
not permit the use of any extensions (because there is no easy and
reliable way to signal rejection to the other side and problems would
be hard to debug). Any compatible extensions to this phase must be
agreed upon during any of the earlier phases.
The Binary Packet Protocol
After the protocol identification strings, both sides only send spe-
cially formatted packets. The packet layout is as follows:
o Packet length: 32 bit unsigned integer, coded as four 8-bit
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bytes, msb first. Gives the length of the packet, not including
the length field and padding. The maximum length of a packet
(not including the length field and padding) is 262144 bytes.
o Padding: 1-8 bytes of random data (or zeroes if not encrypting).
The amount of padding is (8 - (length % 8)) bytes (where %
stands for the modulo operator). The rationale for always hav-
ing some random padding at the beginning of each packet is to
make known plaintext attacks more difficult.
o Packet type: 8-bit unsigned byte. The value 255 is reserved for
future extension.
o Data: binary data bytes, depending on the packet type. The
number of data bytes is the "length" field minus 5.
o Check bytes: 32-bit crc, four 8-bit bytes, msb first. The crc
is the Cyclic Redundancy Check, with the polynomial 0xedb88320,
of the Padding, Packet type, and Data fields. The crc is com-
puted before any encryption.
The packet, except for the length field, may be encrypted using any
of a number of algorithms. The length of the encrypted part (Padding
+ Type + Data + Check) is always a multiple of 8 bytes. Typically
the cipher is used in a chained mode, with all packets chained
together as if it was a single data stream (the length field is never
included in the encryption process). Details of encryption are
described below.
When the session starts, encryption is turned off. Encryption is
enabled after the client has sent the session key. The encryption
algorithm to use is selected by the client.
Packet Compression
If compression is supported (it is an optional feature, see
SSH_CMSG_REQUEST_COMPRESSION below), the packet type and data fields
of the packet are compressed using the gzip deflate algorithm [GZIP].
If compression is in effect, the packet length field indicates the
length of the compressed data, plus 4 for the crc. The amount of
padding is computed from the compressed data, so that the amount of
data to be encrypted becomes a multiple of 8 bytes.
When compressing, the packets (type + data portions) in each direc-
tion are compressed as if they formed a continuous data stream, with
only the current compression block flushed between packets. This
corresponds to the GNU ZLIB library Z_PARTIAL_FLUSH option. The
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compression dictionary is not flushed between packets. The two
directions are compressed independently of each other.
Packet Encryption
The protocol supports several encryption methods. During session
initialization, the server sends a bitmask of all encryption methods
that it supports, and the client selects one of these methods. The
client also generates a 256-bit random session key (32 8-bit bytes)
and sends it to the server.
The encryption methods supported by the current implementation, and
their codes are:
SSH_CIPHER_NONE 0 No encryption
SSH_CIPHER_IDEA 1 IDEA in CFB mode
SSH_CIPHER_DES 2 DES in CBC mode
SSH_CIPHER_3DES 3 Triple-DES in CBC mode
SSH_CIPHER_RC4 5 RC4
All implementations are required to support SSH_CIPHER_3DES. Sup-
porting SSH_CIPHER_IDEA, SSH_CIPHER_RC4, and SSH_CIPHER_NONE is
recommended. Other ciphers may be added at a later time; support
for them is optional.
For encryption, the encrypted portion of the packet is considered a
linear byte stream. The length of the stream is always a multiple of
8. The encrypted portions of consecutive packets (in the same direc-
tion) are encrypted as if they were a continuous buffer (that is, any
initialization vectors are passed from the previous packet to the
next packet). Data in each direction is encrypted independently.
SSH_CIPHER_DES
The key is taken from the first 8 bytes of the session key. The
least significant bit of each byte is ignored. This results in
56 bits of key data. DES [DES] is used in CBC mode. The iv
(initialization vector) is initialized to all zeroes.
SSH_CIPHER_3DES
The variant of triple-DES used here works as follows: there are
three independent DES-CBC ciphers, with independent initializa-
tion vectors. The data (the whole encrypted data stream) is
first encrypted with the first cipher, then decrypted with the
second cipher, and finally encrypted with the third cipher. All
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these operations are performed in CBC mode.
The key for the first cipher is taken from the first 8 bytes of
the session key; the key for the next cipher from the next 8
bytes, and the key for the third cipher from the following 8
bytes. All three initialization vectors are initialized to
zero.
(Note: the variant of 3DES used here differs from some other
descriptions.)
SSH_CIPHER_IDEA
The key is taken from the first 16 bytes of the session key.
IDEA [IDEA] is used in CFB mode. The initialization vector is
initialized to all zeroes.
SSH_CIPHER_RC4
The first 16 bytes of the session key are used as the key for
the server to client direction. The remaining 16 bytes are used
as the key for the client to server direction. This gives
independent 128-bit keys for each direction.
This algorithm is the alleged RC4 cipher posted to the Usenet in
1995. It is widely believed to be equivalent with the original
RSADSI RC4 cipher. This is a very fast algorithm.
Data Type Encodings
The Data field of each packet contains data encoded as described in
this section. There may be several data items; each item is coded as
described here, and their representations are concatenated together
(without any alignment or padding).
Each data type is stored as follows:
8-bit byte
The byte is stored directly as a single byte.
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32-bit unsigned integer
Stored in 4 bytes, msb first.
Arbitrary length binary string
First 4 bytes are the length of the string, msb first (not
including the length itself). The following "length" bytes are
the string value. There are no terminating null characters.
Multiple-precision integer
First 2 bytes are the number of bits in the integer, msb first
(for example, the value 0x00012345 would have 17 bits). The
value zero has zero bits. It is permissible that the number of
bits be larger than the real number of bits.
The number of bits is followed by (bits + 7) / 8 bytes of binary
data, msb first, giving the value of the integer.
TCP/IP Port Number and Other Options
The server listens for connections on TCP/IP port 22.
The client may connect the server from any port. However, if the
client wishes to use any form of .rhosts or /etc/hosts.equiv authen-
tication, it must connect from a privileged port (less than 1024).
For the IP Type of Service field [RFC0791], it is recommended that
interactive sessions (those having a user terminal or forwarding X11
connections) use the IPTOS_LOWDELAY, and non-interactive connections
use IPTOS_THROUGHPUT.
It is recommended that keepalives are used, because otherwise pro-
grams on the server may never notice if the other end of the connec-
tion is rebooted.
Protocol Version Identification
After the socket is opened, the server sends an identification
string, which is of the form "SSH-.-
\n", where and are integers
and specify the protocol version number (not software distribution
version). is server side software version string (max 40
characters); it is not interpreted by the remote side but may be use-
ful for debugging.
The client parses the server's string, and sends a corresponding
string with its own information in response. If the server has lower
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version number, and the client contains special code to emulate it,
the client responds with the lower number; otherwise it responds with
its own number. The server then compares the version number the
client sent with its own, and determines whether they can work
together. The server either disconnects, or sends the first packet
using the binary packet protocol and both sides start working accord-
ing to the lower of the protocol versions.
By convention, changes which keep the protocol compatible with previ-
ous versions keep the same major protocol version; changes that are
not compatible increment the major version (which will hopefully
never happen). The version described in this document is 1.5.
Key Exchange and Server Host Authentication
The first message sent by the server using the packet protocol is
SSH_SMSG_PUBLIC_KEY. It declares the server's host key, server pub-
lic key, supported ciphers, supported authentication methods, and
flags for protocol extensions. It also contains a 64-bit random
number (cookie) that must be returned in the client's reply (to make
IP spoofing more difficult). No encryption is used for this message.
Both sides compute a session id as follows. The modulus of the host
key is interpreted as a byte string (without explicit length field,
with minimum length able to hold the whole value), most significant
byte first. This string is concatenated with the server key inter-
preted the same way. Additionally, the cookie is concatenated with
this. Both sides compute MD5 of the resulting string. The resulting
16 bytes (128 bits) are stored by both parties and are called the
session id.
In other words,
session_id = MD5(hostkey->n || servkey->n || cookie)
The client responds with a SSH_CMSG_SESSION_KEY message, which con-
tains the selected cipher type, a copy of the 64-bit cookie sent by
the server, client's protocol flags, and a session key encrypted with
both the server's host key and server key. No encryption is used for
this message.
The session key is 32 8-bit bytes (a total of 256 random bits gen-
erated by the client). The client first xors the 16 bytes of the
session id with the first 16 bytes of the session key. The resulting
string is then encrypted using the smaller key (one with smaller
modulus), and the result is then encrypted using the other key. The
number of bits in the public modulus of the two keys must differ by
at least 128 bits.
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At each encryption step, a multiple-precision integer is constructed
from the data to be encrypted as follows (the integer is here inter-
preted as a sequence of bytes, msb first; the number of bytes is the
number of bytes needed to represent the modulus).
The most significant byte (which is only partial as the value must be
less than the public modulus, which is never a power of two) is zero.
The next byte contains the value 2 (which stands for public-key
encrypted data in the PKCS standard [PKCS#1]). Then, there are non-
zero random bytes to fill any unused space, a zero byte, and the data
to be encrypted in the least significant bytes, the last byte of the
data in the least significant byte.
This algorithm is used twice. First, it is used to encrypt the 32
random bytes generated by the client to be used as the session key
(xored by the session id). This value is converted to an integer as
described above, and encrypted with RSA using the key with the
smaller modulus. The resulting integer is converted to a byte
stream, msb first. This byte stream is padded and encrypted identi-
cally using the key with the larger modulus.
After the client has sent the session key, it starts to use the
selected algorithm and key for decrypting any received packets, and
for encrypting any sent packets. Separate ciphers are used for dif-
ferent directions (that is, both directions have separate initializa-
tion vectors or other state for the ciphers).
When the server has received the session key message, and has turned
on encryption, it sends a SSH_SMSG_SUCCESS message to the client.
The recommended size of the host key is 1024 bits, and 768 bits for
the server key. The minimum size is 512 bits for the smaller key.
Declaring the User Name
The client then sends a SSH_CMSG_USER message to the server. This
message specifies the user name to log in as.
The server validates that such a user exists, checks whether authen-
tication is needed, and responds with either SSH_SMSG_SUCCESS or
SSH_SMSG_FAILURE. SSH_SMSG_SUCCESS indicates that no authentication
is needed for this user (no password), and authentication phase has
now been completed. SSH_SMSG_FAILURE indicates that authentication
is needed (or the user does not exist).
If the user does not exist, it is recommended that this returns
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failure, but the server keeps reading messages from the client, and
responds to any messages (except SSH_MSG_DISCONNECT, SSH_MSG_IGNORE,
and SSH_MSG_DEBUG) with SSH_SMSG_FAILURE. This way the client cannot
be certain whether the user exists.
Authentication Phase
Provided the server didn't immediately accept the login, an authenti-
cation exchange begins. The client sends messages to the server
requesting different types of authentication in arbitrary order as
many times as desired (however, the server may close the connection
after a timeout). The server always responds with SSH_SMSG_SUCCESS
if it has accepted the authentication, and with SSH_SMSG_FAILURE if
it has denied authentication with the requested method or it does not
recognize the message. Some authentication methods cause an exchange
of further messages before the final result is sent. The authentica-
tion phase ends when the server responds with success.
The recommended value for the authentication timeout (timeout before
disconnecting if no successful authentication has been made) is 5
minutes.
The following authentication methods are currently supported:
SSH_AUTH_RHOSTS 1 .rhosts or /etc/hosts.equiv
SSH_AUTH_RSA 2 pure RSA authentication
SSH_AUTH_PASSWORD 3 password authentication
SSH_AUTH_RHOSTS_RSA 4 .rhosts with RSA host authentication
SSH_AUTH_RHOSTS
This is the authentication method used by rlogin and rsh
[RFC1282].
The client sends SSH_CMSG_AUTH_RHOSTS with the client-side user
name as an argument.
The server checks whether to permit authentication. On UNIX
systems, this is usually done by checking /etc/hosts.equiv, and
.rhosts in the user's home directory. The connection must come
from a privileged port.
It is recommended that the server checks that there are no IP
options (such as source routing) specified for the socket before
accepting this type of authentication. The client host name
should be reverse-mapped and then forward mapped to ensure that
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it has the proper IP-address.
This authentication method trusts the remote host (root on the
remote host can pretend to be any other user on that host), the
name services, and partially the network: anyone who can see
packets coming out from the server machine can do IP-spoofing
and pretend to be any machine; however, the protocol prevents
blind IP-spoofing (which used to be possible with rlogin).
Many sites probably want to disable this authentication method
because of the fundamental insecurity of conventional .rhosts or
/etc/hosts.equiv authentication when faced with spoofing. It is
recommended that this method not be supported by the server by
default.
SSH_AUTH_RHOSTS_RSA
In addition to conventional .rhosts and hosts.equiv authentica-
tion, this method additionally requires that the client host be
authenticated using RSA.
The client sends SSH_CMSG_AUTH_RHOSTS_RSA specifying the
client-side user name, and the public host key of the client
host.
The server first checks if normal .rhosts or /etc/hosts.equiv
authentication would be accepted, and if not, responds with
SSH_SMSG_FAILURE. Otherwise, it checks whether it knows the
host key for the client machine (using the same name for the
host that was used for checking the .rhosts and /etc/hosts.equiv
files). If it does not know the RSA key for the client, access
is denied and SSH_SMSG_FAILURE is sent.
If the server knows the host key of the client machine, it veri-
fies that the given host key matches that known for the client.
If not, access is denied and SSH_SMSG_FAILURE is sent.
The server then sends a SSH_SMSG_AUTH_RSA_CHALLENGE message con-
taining an encrypted challenge for the client. The challenge is
32 8-bit random bytes (256 bits). When encrypted, the highest
(partial) byte is left as zero, the next byte contains the value
2, the following are non-zero random bytes, followed by a zero
byte, and the challenge put in the remaining bytes. This is
then encrypted using RSA with the client host's public key.
(The padding and encryption algorithm is the same as that used
for the session key.)
The client decrypts the challenge using its private host key,
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concatenates this with the session id, and computes an MD5
checksum of the resulting 48 bytes. The MD5 output is returned
as 16 bytes in a SSH_CMSG_AUTH_RSA_RESPONSE message. (MD5 is
used to deter chosen plaintext attacks against RSA; the session
id binds it to a specific session).
The server verifies that the MD5 of the decrypted challenge
returned by the client matches that of the original value, and
sends SSH_SMSG_SUCCESS if so. Otherwise it sends
SSH_SMSG_FAILURE and refuses the authentication attempt.
This authentication method trusts the client side machine in
that root on that machine can pretend to be any user on that
machine. Additionally, it trusts the client host key. The name
and/or IP address of the client host is only used to select the
public host key. The same host name is used when scanning
.rhosts or /etc/hosts.equiv and when selecting the host key. It
would in principle be possible to eliminate the host name
entirely and substitute it directly by the host key. IP and/or
DNS [RFC1034] spoofing can only be used to pretend to be a host
for which the attacker has the private host key.
SSH_AUTH_RSA
The idea behind RSA authentication is that the server recognizes
the public key offered by the client, generates a random chal-
lenge, and encrypts the challenge with the public key. The
client must then prove that it has the corresponding private key
by decrypting the challenge.
The client sends SSH_CMSG_AUTH_RSA with public key modulus (n)
as an argument.
The server may respond immediately with SSH_SMSG_FAILURE if it
does not permit authentication with this key. Otherwise it gen-
erates a challenge, encrypts it using the user's public key
(stored on the server and identified using the modulus), and
sends SSH_SMSG_AUTH_RSA_CHALLENGE with the challenge (mp-int) as
an argument.
The challenge is 32 8-bit random bytes (256 bits). When
encrypted, the highest (partial) byte is left as zero, the next
byte contains the value 2, the following are non-zero random
bytes, followed by a zero byte, and the challenge put in the
remaining bytes. This is then encrypted with the public key.
(The padding and encryption algorithm is the same as that used
for the session key.)
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The client decrypts the challenge using its private key, con-
catenates it with the session id, and computes an MD5 checksum
of the resulting 48 bytes. The MD5 output is returned as 16
bytes in a SSH_CMSG_AUTH_RSA_RESPONSE message. (Note that the
MD5 is necessary to avoid chosen plaintext attacks against RSA;
the session id binds it to a specific session.)
The server verifies that the MD5 of the decrypted challenge
returned by the client matches that of the original value, and
sends SSH_SMSG_SUCCESS if so. Otherwise it sends
SSH_SMSG_FAILURE and refuses the authentication attempt.
This authentication method does not trust the remote host, the
network, name services, or anything else. Authentication is
based solely on the possession of the private identification
keys. Anyone in possession of the private keys can log in, but
nobody else.
The server may have additional requirements for a successful
authentiation. For example, to limit damage due to a comprom-
ised RSA key, a server might restrict access to a limited set of
hosts.
SSH_AUTH_PASSWORD
The client sends a SSH_CMSG_AUTH_PASSWORD message with the plain
text password. (Note that even though the password is plain
text inside the message, it is normally encrypted by the packet
mechanism.)
The server verifies the password, and sends SSH_SMSG_SUCCESS if
authentication was accepted and SSH_SMSG_FAILURE otherwise.
Note that the password is read from the user by the client; the
user never interacts with a login program.
This authentication method does not trust the remote host, the
network, name services or anything else. Authentication is
based solely on the possession of the password. Anyone in pos-
session of the password can log in, but nobody else.
Preparatory Operations
After successful authentication, the server waits for a request from
the client, processes the request, and responds with SSH_SMSG_SUCCESS
whenever a request has been successfully processed. If it receives a
message that it does not recognize or it fails to honor a request, it
returns SSH_SMSG_FAILURE. It is expected that new message types
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might be added to this phase in future.
The following messages are currently defined for this phase.
SSH_CMSG_REQUEST_COMPRESSION
Requests that compression be enabled for this session. A gzip-
compatible compression level (1-9) is passed as an argument.
SSH_CMSG_REQUEST_PTY
Requests that a pseudo terminal device be allocated for this
session. The user terminal type and terminal modes are supplied
as arguments.
SSH_CMSG_X11_REQUEST_FORWARDING
Requests forwarding of X11 connections from the remote machine
to the local machine over the secure channel. Causes an
internet-domain socket to be allocated and the DISPLAY variable
to be set on the server. X11 authentication data is automati-
cally passed to the server, and the client may implement spoof-
ing of authentication data for added security. The authentica-
tion data is passed as arguments.
SSH_CMSG_PORT_FORWARD_REQUEST
Requests forwarding of a TCP/IP port on the server host over the
secure channel. What happens is that whenever a connection is
made to the port on the server, a connection will be made from
the client end to the specified host/port. Any user can forward
unprivileged ports; only the root can forward privileged ports
(as determined by authentication done earlier).
SSH_CMSG_AGENT_REQUEST_FORWARDING
Requests forwarding of the connection to the authentication
agent.
SSH_CMSG_EXEC_SHELL
Starts a shell (command interpreter) for the user, and moves
into interactive session mode.
SSH_CMSG_EXEC_CMD
Executes the given command (actually " -c " or
equivalent) for the user, and moves into interactive session
mode.
Interactive Session and Exchange of Data
During the interactive session, any data written by the shell or com-
mand running on the server machine is forwarded to stdin or stderr on
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the client machine, and any input available from stdin on the client
machine is forwarded to the program on the server machine.
All exchange is asynchronous; either side can send at any time, and
there are no acknowledgements (TCP/IP already provides reliable tran-
sport, and the packet protocol protects against tampering or IP
spoofing).
When the client receives EOF from its standard input, it will send
SSH_CMSG_EOF; however, this in no way terminates the exchange. The
exchange terminates and interactive mode is left when the server
sends SSH_SMSG_EXITSTATUS to indicate that the client program has
terminated. Alternatively, either side may disconnect at any time by
sending SSH_MSG_DISCONNECT or closing the connection.
The server may send any of the following messages:
SSH_SMSG_STDOUT_DATA
Data written to stdout by the program running on the server.
The data is passed as a string argument. The client writes this
data to stdout.
SSH_SMSG_STDERR_DATA
Data written to stderr by the program running on the server.
The data is passed as a string argument. The client writes this
data to stderr. (Note that if the program is running on a tty,
it is not possible to separate stdout and stderr data, and all
data will be sent as stdout data.)
SSH_SMSG_EXITSTATUS
Indicates that the shell or command has exited. Exit status is
passed as an integer argument. This message causes termination
of the interactive session.
SSH_SMSG_AGENT_OPEN
Indicates that someone on the server side is requesting a con-
nection to the authentication agent. The server-side channel
number is passed as an argument. The client must respond with
either SSH_CHANNEL_OPEN_CONFIRMATION or
SSH_CHANNEL_OPEN_FAILURE.
SSH_SMSG_X11_OPEN
Indicates that a connection has been made to the X11 socket on
the server side and should be forwarded to the real X server.
An integer argument indicates the channel number allocated for
this connection on the server side. The client should send back
either SSH_MSG_CHANNEL_OPEN_CONFIRMATION or
SSH_MSG_CHANNEL_OPEN_FAILURE with the same server side channel
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number.
SSH_MSG_PORT_OPEN
Indicates that a connection has been made to a port on the
server side for which forwarding has been requested. Arguments
are server side channel number, host name to connect to, and
port to connect to. The client should send back either
SSH_MSG_CHANNEL_OPEN_CONFIRMATION or
SSH_MSG_CHANNEL_OPEN_FAILURE with the same server side channel
number.
SSH_MSG_CHANNEL_OPEN_CONFIRMATION
This is sent by the server to indicate that it has opened a con-
nection as requested in a previous message. The first argument
indicates the client side channel number, and the second argu-
ment is the channel number that the server has allocated for
this connection.
SSH_MSG_CHANNEL_OPEN_FAILURE
This is sent by the server to indicate that it failed to open a
connection as requested in a previous message. The client-side
channel number is passed as an argument. The client will close
the descriptor associated with the channel and free the channel.
SSH_MSG_CHANNEL_DATA
This packet contains data for a channel from the server. The
first argument is the client-side channel number, and the second
argument (a string) is the data.
SSH_MSG_CHANNEL_CLOSE
This is sent by the server to indicate that whoever was in the
other end of the channel has closed it. The argument is the
client side channel number. The client will let all buffered
data in the channel to drain, and when ready, will close the
socket, free the channel, and send the server a
SSH_MSG_CHANNEL_CLOSE_CONFIRMATION message for the channel.
SSH_MSG_CHANNEL_CLOSE_CONFIRMATION
This is send by the server to indicate that a channel previously
closed by the client has now been closed on the server side as
well. The argument indicates the client channel number. The
client frees the channel.
The client may send any of the following messages:
SSH_CMSG_STDIN_DATA
This is data to be sent as input to the program running on the
server. The data is passed as a string.
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SSH_CMSG_EOF
Indicates that the client has encountered EOF while reading
standard input. The server will allow any buffered input data
to drain, and will then close the input to the program.
SSH_CMSG_WINDOW_SIZE
Indicates that window size on the client has been changed. The
server updates the window size of the tty and causes SIGWINCH to
be sent to the program. The new window size is passed as four
integer arguments: row, col, xpixel, ypixel.
SSH_MSG_PORT_OPEN
Indicates that a connection has been made to a port on the
client side for which forwarding has been requested. Arguments
are client side channel number, host name to connect to, and
port to connect to. The server should send back either
SSH_MSG_CHANNEL_OPEN_CONFIRMATION or
SSH_MSG_CHANNEL_OPEN_FAILURE with the same client side channel
number.
SSH_MSG_CHANNEL_OPEN_CONFIRMATION
This is sent by the client to indicate that it has opened a con-
nection as requested in a previous message. The first argument
indicates the server side channel number, and the second argu-
ment is the channel number that the client has allocated for
this connection.
SSH_MSG_CHANNEL_OPEN_FAILURE
This is sent by the client to indicate that it failed to open a
connection as requested in a previous message. The server side
channel number is passed as an argument. The server will close
the descriptor associated with the channel and free the channel.
SSH_MSG_CHANNEL_DATA
This packet contains data for a channel from the client. The
first argument is the server side channel number, and the second
argument (a string) is the data.
SSH_MSG_CHANNEL_CLOSE
This is sent by the client to indicate that whoever was in the
other end of the channel has closed it. The argument is the
server channel number. The server will allow buffered data to
drain, and when ready, will close the socket, free the channel,
and send the client a SSH_MSG_CHANNEL_CLOSE_CONFIRMATION message
for the channel.
SSH_MSG_CHANNEL_CLOSE_CONFIRMATION
This is send by the client to indicate that a channel previously
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closed by the server has now been closed on the client side as
well. The argument indicates the server channel number. The
server frees the channel.
Any unsupported messages during interactive mode cause the connection
to be terminated with SSH_MSG_DISCONNECT and an error message. Com-
patible protocol upgrades should agree about any extensions during
the preparation phase or earlier.
Termination of the Connection
Normal termination of the connection is always initiated by the
server by sending SSH_SMSG_EXITSTATUS after the program has exited.
The client responds to this message by sending
SSH_CMSG_EXIT_CONFIRMATION and closes the socket; the server then
closes the socket. There are two purposes for the confirmation: some
systems may lose previously sent data when the socket is closed, and
closing the client side first causes any TCP/IP TIME_WAIT [RFC0793]
waits to occur on the client side, not consuming server resources.
If the program terminates due to a signal, the server will send
SSH_MSG_DISCONNECT with an appropriate message. If the connection is
closed, all file descriptors to the program will be closed and the
server will exit. If the program runs on a tty, the kernel sends it
the SIGHUP signal when the pty master side is closed.
Protocol Flags
Both the server and the client pass 32 bits of protocol flags to the
other side. The flags are intended for compatible protocol exten-
sion; the server first announces which added capabilities it sup-
ports, and the client then sends the capabilities that it supports.
The following flags are currently defined (the values are bit masks):
1 SSH_PROTOFLAG_SCREEN_NUMBER
This flag can only be sent by the client. It indicates that the
X11 forwarding requests it sends will include the screen number.
2 SSH_PROTOFLAG_HOST_IN_FWD_OPEN
If both sides specify this flag, SSH_SMSG_X11_OPEN and
SSH_MSG_PORT_OPEN messages will contain an additional field con-
taining a description of the host at the other end of the con-
nection.
Detailed Description of Packet Types and Formats
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The supported packet types and the corresponding message numbers are
given in the following table. Messages with _MSG_ in their name may
be sent by either side. Messages with _CMSG_ are only sent by the
client, and messages with _SMSG_ only by the server.
A packet may contain additional data after the arguments specified
below. Any such data should be ignored by the receiver. However, it
is recommended that no such data be stored without good reason.
(This helps build compatible extensions.)
0 SSH_MSG_NONE
This code is reserved. This message type is never sent.
1 SSH_MSG_DISCONNECT
string Cause of disconnection
This message may be sent by either party at any time. It causes
the immediate disconnection of the connection. The message is
intended to be displayed to a human, and describes the reason
for disconnection.
2 SSH_SMSG_PUBLIC_KEY
8 bytes anti_spoofing_cookie
32-bit int server_key_bits
mp-int server_key_public_exponent
mp-int server_key_public_modulus
32-bit int host_key_bits
mp-int host_key_public_exponent
mp-int host_key_public_modulus
32-bit int protocol_flags
32-bit int supported_ciphers_mask
32-bit int supported_authentications_mask
Sent as the first message by the server. This message gives the
server's host key, server key, protocol flags (intended for com-
patible protocol extension), supported_ciphers_mask (which is
the bitwise or of (1 << cipher_number), where << is the left
shift operator, for all supported ciphers), and
supported_authentications_mask (which is the bitwise or of (1 <<
authentication_type) for all supported authentication types).
The anti_spoofing_cookie is 64 random bits, and must be sent
back verbatim by the client in its reply. It is used to make
IP-spoofing more difficult (encryption and host keys are the
real defense against spoofing).
3 SSH_CMSG_SESSION_KEY
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1 byte cipher_type (must be one of the supported values)
8 bytes anti_spoofing_cookie (must match data sent by the server)
mp-int double-encrypted session key
32-bit int protocol_flags
Sent by the client as the first message in the session. Selects
the cipher to use, and sends the encrypted session key to the
server. The anti_spoofing_cookie must be the same bytes that
were sent by the server. Protocol_flags is intended for nego-
tiating compatible protocol extensions.
4 SSH_CMSG_USER
string user login name on server
Sent by the client to begin authentication. Specifies the user
name on the server to log in as. The server responds with
SSH_SMSG_SUCCESS if no authentication is needed for this user,
or SSH_SMSG_FAILURE if authentication is needed (or the user
does not exist). [Note to the implementator: the user name is
of arbitrary size. The implementation must be careful not to
overflow internal buffers.]
5 SSH_CMSG_AUTH_RHOSTS
string client-side user name
Requests authentication using /etc/hosts.equiv and .rhosts (or
equivalent mechanisms). This authentication method is normally
disabled in the server because it is not secure (but this is the
method used by rsh and rlogin). The server responds with
SSH_SMSG_SUCCESS if authentication was successful, and
SSH_SMSG_FAILURE if access was not granted. The server should
check that the client side port number is less than 1024 (a
privileged port), and immediately reject authentication if it is
not. Supporting this authentication method is optional. This
method should normally not be enabled in the server because it
is not safe. (However, not enabling this only helps if rlogind
and rshd are disabled.)
6 SSH_CMSG_AUTH_RSA
mp-int identity_public_modulus
Requests authentication using pure RSA authentication. The
server checks if the given key is permitted to log in, and if
so, responds with SSH_SMSG_AUTH_RSA_CHALLENGE. Otherwise, it
responds with SSH_SMSG_FAILURE. The client often tries several
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different keys in sequence until one supported by the server is
found. Authentication is accepted if the client gives the
correct response to the challenge. The server is free to add
other criteria for authentication, such as a requirement that
the connection must come from a certain host. Such additions
are not visible at the protocol level. Supporting this authen-
tication method is optional but recommended.
7 SSH_SMSG_AUTH_RSA_CHALLENGE
mp-int encrypted challenge
Presents an RSA authentication challenge to the client. The
challenge is a 256-bit random value encrypted as described else-
where in this document. The client must decrypt the challenge
using the RSA private key, compute MD5 of the challenge plus
session id, and send back the resulting 16 bytes using
SSH_CMSG_AUTH_RSA_RESPONSE.
8 SSH_CMSG_AUTH_RSA_RESPONSE
16 bytes MD5 of decrypted challenge
This message is sent by the client in response to an RSA chal-
lenge. The MD5 checksum is returned instead of the decrypted
challenge to deter known-plaintext attacks against the RSA key.
The server responds to this message with either SSH_SMSG_SUCCESS
or SSH_SMSG_FAILURE.
9 SSH_CMSG_AUTH_PASSWORD
string plain text password
Requests password authentication using the given password. Note
that even though the password is plain text inside the packet,
the whole packet is normally encrypted by the packet layer. It
would not be possible for the client to perform password
encryption/hashing, because it cannot know which kind of
encryption/hashing, if any, the server uses. The server
responds to this message with SSH_SMSG_SUCCESS or
SSH_SMSG_FAILURE.
10 SSH_CMSG_REQUEST_PTY
string TERM environment variable value (e.g. vt100)
32-bit int terminal height, rows (e.g., 24)
32-bit int terminal width, columns (e.g., 80)
32-bit int terminal width, pixels (0 if no graphics) (e.g., 480)
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32-bit int terminal height, pixels (0 if no graphics) (e.g., 640)
n bytes tty modes encoded in binary
Requests a pseudo-terminal to be allocated for this command.
This message can be used regardless of whether the session will
later execute the shell or a command. If a pty has been
requested with this message, the shell or command will run on a
pty. Otherwise it will communicate with the server using pipes,
sockets or some other similar mechanism.
The terminal type gives the type of the user's terminal. In the
UNIX environment it is passed to the shell or command in the
TERM environment variable.
The width and height values give the initial size of the user's
terminal or window. All values can be zero if not supported by
the operating system. The server will pass these values to the
kernel if supported.
Terminal modes are encoded into a byte stream in a portable for-
mat. The exact format is described later in this document.
The server responds to the request with either SSH_SMSG_SUCCESS
or SSH_SMSG_FAILURE. If the server does not have the concept of
pseudo terminals, it should return success if it is possible to
execute a shell or a command so that it looks to the client as
if it was running on a pseudo terminal.
11 SSH_CMSG_WINDOW_SIZE
32-bit int terminal height, rows
32-bit int terminal width, columns
32-bit int terminal width, pixels
32-bit int terminal height, pixels
This message can only be sent by the client during the interac-
tive session. This indicates that the size of the user's window
has changed, and provides the new size. The server will update
the kernel's notion of the window size, and a SIGWINCH signal or
equivalent will be sent to the shell or command (if supported by
the operating system).
12 SSH_CMSG_EXEC_SHELL
(no arguments)
Starts a shell (command interpreter), and enters interactive
session mode.
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13 SSH_CMSG_EXEC_CMD
string command to execute
Starts executing the given command, and enters interactive ses-
sion mode. On UNIX, the command is run as " -c ", where is the user's login shell.
14 SSH_SMSG_SUCCESS
(no arguments)
This message is sent by the server in response to the session
key, a successful authentication request, and a successfully
completed preparatory operation.
15 SSH_SMSG_FAILURE
(no arguments)
This message is sent by the server in response to a failed
authentication operation to indicate that the user has not yet
been successfully authenticated, and in response to a failed
preparatory operation. This is also sent in response to an
authentication or preparatory operation request that is not
recognized or supported.
16 SSH_CMSG_STDIN_DATA
string data
Delivers data from the client to be supplied as input to the
shell or program running on the server side. This message can
only be used in the interactive session mode. No acknowledge-
ment is sent for this message.
17 SSH_SMSG_STDOUT_DATA
string data
Delivers data from the server that was read from the standard
output of the shell or program running on the server side. This
message can only be used in the interactive session mode. No
acknowledgement is sent for this message.
18 SSH_SMSG_STDERR_DATA
string data
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Delivers data from the server that was read from the standard
error of the shell or program running on the server side. This
message can only be used in the interactive session mode. No
acknowledgement is sent for this message.
19 SSH_CMSG_EOF
(no arguments)
This message is sent by the client to indicate that EOF has been
reached on the input. Upon receiving this message, and after
all buffered input data has been sent to the shell or program,
the server will close the input file descriptor to the program.
This message can only be used in the interactive session mode.
No acknowledgement is sent for this message.
20 SSH_SMSG_EXITSTATUS
32-bit int exit status of the command
Returns the exit status of the shell or program after it has
exited. The client should respond with
SSH_CMSG_EXIT_CONFIRMATION when it has received this message.
This will be the last message sent by the server. If the pro-
gram being executed dies with a signal instead of exiting nor-
mally, the server should terminate the session with
SSH_MSG_DISCONNECT (which can be used to pass a human-readable
string indicating that the program died due to a signal) instead
of using this message.
21 SSH_MSG_CHANNEL_OPEN_CONFIRMATION
32-bit int remote_channel
32-bit int local_channel
This is sent in response to any channel open request if the
channel has been successfully opened. Remote_channel is the
channel number received in the initial open request;
local_channel is the channel number the side sending this mes-
sage has allocated for the channel. Data can be transmitted on
the channel after this message.
22 SSH_MSG_CHANNEL_OPEN_FAILURE
32-bit int remote_channel
This message indicates that an earlier channel open request by
the other side has failed or has been denied. Remote_channel is
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the channel number given in the original request.
23 SSH_MSG_CHANNEL_DATA
32-bit int remote_channel
string data
Data is transmitted in a channel in these messages. A channel
is bidirectional, and both sides can send these messages. There
is no acknowledgement for these messages. It is possible that
either side receives these messages after it has sent
SSH_MSG_CHANNEL_CLOSE for the channel. These messages cannot be
received after the party has sent or received
SSH_MSG_CHANNEL_CLOSE_CONFIRMATION.
24 SSH_MSG_CHANNEL_CLOSE
32-bit int remote_channel
When a channel is closed at one end of the connection, that side
sends this message. Upon receiving this message, the channel
should be closed. When this message is received, if the channel
is already closed (the receiving side has sent this message for
the same channel earlier), the channel is freed and no further
action is taken; otherwise the channel is freed and
SSH_MSG_CHANNEL_CLOSE_CONFIRMATION is sent in response. (It is
possible that the channel is closed simultaneously at both
ends.)
25 SSH_MSG_CHANNEL_CLOSE_CONFIRMATION
32-bit int remote_channel
This message is sent in response to SSH_MSG_CHANNEL_CLOSE unless
the channel was already closed. When this message is sent or
received, the channel is freed.
26 (OBSOLETED; was unix-domain X11 forwarding)
27 SSH_SMSG_X11_OPEN
32-bit int local_channel
string originator_string (see below)
This message can be sent by the server during the interactive
session mode to indicate that a client has connected the fake X
server. Local_channel is the channel number that the server has
allocated for the connection. The client should try to open a
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connection to the real X server, and respond with
SSH_MSG_CHANNEL_OPEN_CONFIRMATION or
SSH_MSG_CHANNEL_OPEN_FAILURE.
The field originator_string is present if both sides specified
SSH_PROTOFLAG_HOST_IN_FWD_OPEN in the protocol flags. It con-
tains a description of the host originating the connection.
28 SSH_CMSG_PORT_FORWARD_REQUEST
32-bit int server_port
string host_to_connect
32-bit int port_to_connect
Sent by the client in the preparatory phase, this message
requests that server_port on the server machine be forwarded
over the secure channel to the client machine, and from there to
the specified host and port. The server should start listening
on the port, and send SSH_MSG_PORT_OPEN whenever a connection is
made to it. Supporting this message is optional, and the server
is free to reject any forward request. For example, it is
highly recommended that unless the user has been authenticated
as root, forwarding any privileged port numbers (below 1024) is
denied.
29 SSH_MSG_PORT_OPEN
32-bit int local_channel
string host_name
32-bit int port
string originator_string (see below)
Sent by either party in interactive session mode, this message
indicates that a connection has been opened to a forwarded
TCP/IP port. Local_channel is the channel number that the send-
ing party has allocated for the connection. Host_name is the
host the connection should be be forwarded to, and the port is
the port on that host to connect. The receiving party should
open the connection, and respond with
SSH_MSG_CHANNEL_OPEN_CONFIRMATION or
SSH_MSG_CHANNEL_OPEN_FAILURE. It is recommended that the
receiving side check the host_name and port for validity to
avoid compromising local security by compromised remote side
software. Particularly, it is recommended that the client per-
mit connections only to those ports for which it has requested
forwarding with SSH_CMSG_PORT_FORWARD_REQUEST.
The field originator_string is present if both sides specified
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SSH_PROTOFLAG_HOST_IN_FWD_OPEN in the protocol flags. It con-
tains a description of the host originating the connection.
30 SSH_CMSG_AGENT_REQUEST_FORWARDING
(no arguments)
Requests that the connection to the authentication agent be for-
warded over the secure channel. The method used by clients to
contact the authentication agent within each machine is imple-
mentation and machine dependent. If the server accepts this
request, it should arrange that any clients run from this ses-
sion will actually contact the server program when they try to
contact the authentication agent. The server should then send a
SSH_SMSG_AGENT_OPEN to open a channel to the agent, and the
client should forward the connection to the real authentication
agent. Supporting this message is optional.
31 SSH_SMSG_AGENT_OPEN
32-bit int local_channel
Sent by the server in interactive session mode, this message
requests opening a channel to the authentication agent. The
client should open a channel, and respond with either
SSH_MSG_CHANNEL_OPEN_CONFIRMATION or
SSH_MSG_CHANNEL_OPEN_FAILURE.
32 SSH_MSG_IGNORE
string data
Either party may send this message at any time. This message,
and the argument string, is silently ignored. This message
might be used in some implementations to make traffic analysis
more difficult. This message is not currently sent by the
implementation, but all implementations are required to recog-
nize and ignore it.
33 SSH_CMSG_EXIT_CONFIRMATION
(no arguments)
Sent by the client in response to SSH_SMSG_EXITSTATUS. This is
the last message sent by the client.
34 SSH_CMSG_X11_REQUEST_FORWARDING
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string x11_authentication_protocol
string x11_authentication_data
32-bit int screen number (if SSH_PROTOFLAG_SCREEN_NUMBER)
Sent by the client during the preparatory phase, this message
requests that the server create a fake X11 display and set the
DISPLAY environment variable accordingly. An internet-domain
display is preferable. The given authentication protocol and
the associated data should be recorded by the server so that it
is used as authentication on connections (e.g., in .Xauthority).
The authentication protocol must be one of the supported X11
authentication protocols, e.g., "MIT-MAGIC-COOKIE-1". Authenti-
cation data must be a lowercase hex string of even length. Its
interpretation is protocol dependent. The data is in a format
that can be used with e.g. the xauth program. Supporting this
message is optional.
The client is permitted (and recommended) to generate fake
authentication information and send fake information to the
server. This way, a corrupt server will not have access to the
user's terminal after the connection has terminated. The
correct authorization codes will also not be left hanging around
in files on the server (many users keep the same X session for
months, thus protecting the authorization data becomes impor-
tant).
X11 authentication spoofing works by initially sending fake
(random) authentication data to the server, and interpreting the
first packet sent by the X11 client after the connection has
been opened. The first packet contains the client's authentica-
tion. If the packet contains the correct fake data, it is
replaced by the client by the correct authentication data, and
then sent to the X server.
35 SSH_CMSG_AUTH_RHOSTS_RSA
string clint-side user name
32-bit int client_host_key_bits
mp-int client_host_key_public_exponent
mp-int client_host_key_public_modulus
Requests authentication using /etc/hosts.equiv and .rhosts (or
equivalent) together with RSA host authentication. The server
should check that the client side port number is less than 1024
(a privileged port), and immediately reject authentication if it
is not. The server responds with SSH_SMSG_FAILURE or
SSH_SMSG_AUTH_RSA_CHALLENGE. The client must respond to the
challenge with the proper SSH_CMSG_AUTH_RSA_RESPONSE. The
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server then responds with success if access was granted, or
failure if the client gave a wrong response. Supporting this
authentication method is optional but recommended in most
environments.
36 SSH_MSG_DEBUG
string debugging message sent to the other side
This message may be sent by either party at any time. It is
used to send debugging messages that may be informative to the
user in solving various problems. For example, if authentica-
tion fails because of some configuration error (e.g., incorrect
permissions for some file), it can be very helpful for the user
to make the cause of failure available. On the other hand, one
should not make too much information available for security rea-
sons. It is recommended that the client provides an option to
display the debugging information sent by the sender (the user
probably does not want to see it by default). The server can
log debugging data sent by the client (if any). Either party is
free to ignore any received debugging data. Every implementa-
tion must be able to receive this message, but no implementation
is required to send these.
37 SSH_CMSG_REQUEST_COMPRESSION
32-bit int gzip compression level (1-9)
This message can be sent by the client in the preparatory opera-
tions phase. The server responds with SSH_SMSG_FAILURE if it
does not support compression or does not want to compress; it
responds with SSH_SMSG_SUCCESS if it accepted the compression
request. In the latter case the response to this packet will
still be uncompressed, but all further packets in either direc-
tion will be compressed by gzip.
38 SSH_CMSG_MAX_PACKET_SIZE
32-bit int maximum packet size, bytes (4096-1024k)
This message can be sent by the client in the preparatory opera-
tions phase. The server responds with SSH_SMSG_FAILURE if it
does not support limiting packet size, or with SSH_SMSG_SUCCESS
if it has limited the maximum packet size (as determined by the
value in the size field) to the specified value.
39 SSH_CMSG_AUTH_TIS
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(no arguments)
This message starts TIS authentication. The server responds with
SSH_SMSG_FAILURE or SSH_SMSG_AUTH_TIS_CHALLENGE.
40 SSH_SMSG_AUTH_TIS_CHALLENGE
string tis challenge
Server sends TIS challenge to user and client should show it to
user and ask for response, which is sent back using
SSH_CMSG_AUTH_TIS_RESPONSE message.
41 SSH_CMSG_AUTH_TIS_RESPONSE
string user response to tis challenge
When client receives SSH_SMSG_AUTH_TIS_CHALLENGE and ask users
response to challenge it sends it back this message. The server
answers with SSH_SMSG_FAILURE or SSH_SMSG_SUCCESS.
42 SSH_CMSG_AUTH_KERBEROS
string authentication info
Client sends authentication info to server, which replies with
SSH_SMSG_AUTH_KERBEROS_RESPONSE message having correct response
data encrypted with the session key.
43 SSH_SMSG_AUTH_KERBEROS_RESPONSE
string response data
Server replies to SSH_CMSG_AUTH_KERBEROS message with this mes-
sage so that the response data is encrypted with session key.
44 SSH_CMSG_HAVE_KERBEROS_TGT
string kerberos credentials
Client sends kerberos credentials to server and the server
replies with SSH_SMSG_SUCCESS or SSH_SMSG_FAILURE.
Encoding of Terminal Modes
Terminal modes (as passed in SSH_CMSG_REQUEST_PTY) are encoded into a
byte stream. It is intended that the coding be portable across
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different environments.
The tty mode description is a stream of bytes. The stream consists
of opcode-argument pairs. It is terminated by opcode TTY_OP_END (0).
Opcodes 1-127 have one-byte arguments. Opcodes 128-159 have 32-bit
integer arguments (stored msb first). Opcodes 160-255 are not yet
defined, and cause parsing to stop (they should only be used after
any other data).
The client puts in the stream any modes it knows about, and the
server ignores any modes it does not know about. This allows some
degree of machine-independence, at least between systems that use a
POSIX-like [POSIX] tty interface. The protocol can support other
systems as well, but the client may need to fill reasonable values
for a number of parameters so the server pty gets set to a reasonable
mode (the server leaves all unspecified mode bits in their default
values, and only some combinations make sense).
The following opcodes have been defined. The naming of opcodes
mostly follows the POSIX terminal mode flags.
0 TTY_OP_END
Indicates end of options.
1 VINTR
Interrupt character; 255 if none. Similarly for the other char-
acters. Not all of these characters are supported on all sys-
tems.
2 VQUIT
The quit character (sends SIGQUIT signal on UNIX systems).
3 VERASE
Erase the character to left of the cursor.
4 VKILL
Kill the current input line.
5 VEOF
End-of-file character (sends EOF from the terminal).
6 VEOL
End-of-line character in addition to carriage return and/or
linefeed.
7 VEOL2
Additional end-of-line character.
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8 VSTART
Continues paused output (normally ^Q).
9 VSTOP
Pauses output (^S).
10 VSUSP
Suspends the current program.
11 VDSUSP
Another suspend character.
12 VREPRINT
Reprints the current input line.
13 VWERASE
Erases a word left of cursor.
14 VLNEXT
More special input characters; these are probably not supported
on most systems.
15 VFLUSH
16 VSWTCH
17 VSTATUS
18 VDISCARD
30 IGNPAR
The ignore parity flag. The next byte should be 0 if this flag
is not set, and 1 if it is set.
31 PARMRK
More flags. The exact definitions can be found in the POSIX
standard.
32 INPCK
33 ISTRIP
34 INLCR
35 IGNCR
36 ICRNL
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37 IUCLC
38 IXON
39 IXANY
40 IXOFF
41 IMAXBEL
50 ISIG
51 ICANON
52 XCASE
53 ECHO
54 ECHOE
55 ECHOK
56 ECHONL
57 NOFLSH
58 TOSTOP
59 IEXTEN
60 ECHOCTL
61 ECHOKE
62 PENDIN
70 OPOST
71 OLCUC
72 ONLCR
73 OCRNL
74 ONOCR
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75 ONLRET
90 CS7
91 CS8
92 PARENB
93 PARODD
192 TTY_OP_ISPEED
Specifies the input baud rate in bits per second (as a 32-bit
int, msb first).
193 TTY_OP_OSPEED
Specifies the output baud rate in bits per second (as a 32-bt
int, msb first).
The Authentication Agent Protocol
The authentication agent is a program that can be used to hold RSA
authentication keys for the user (in future, it might hold data for
other authentication types as well). An authorized program can send
requests to the agent to generate a proper response to an RSA chal-
lenge. How the connection is made to the agent (or its representa-
tive) inside a host and how access control is done inside a host is
implementation-dependent; however, how it is forwarded and how one
interacts with it is specified in this protocol. The connection to
the agent is normally automatically forwarded over the secure chan-
nel.
A program that wishes to use the agent first opens a connection to
its local representative (typically, the agent itself or an SSH
server). It then writes a request to the connection, and waits for
response. It is recommended that at least five minutes of timeout
are provided waiting for the agent to respond to an authentication
challenge (this gives sufficient time for the user to cut-and-paste
the challenge to a separate machine, perform the computation there,
and cut-and-paste the result back if so desired).
Messages sent to and by the agent are in the following format:
4 bytes Length, msb first. Does not include length itself.
1 byte Packet type. The value 255 is reserved for future extensions.
data Any data, depending on packet type. Encoding as in the ssh packet
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protocol.
The following message types are currently defined:
1 SSH_AGENTC_REQUEST_RSA_IDENTITIES
(no arguments)
Requests the agent to send a list of all RSA keys for which it
can answer a challenge.
2 SSH_AGENT_RSA_IDENTITIES_ANSWER
32-bit int howmany
howmany times:
32-bit int bits
mp-int public exponent
mp-int public modulus
string comment
The agent sends this message in response to the to
SSH_AGENTC_REQUEST_RSA_IDENTITIES. The answer lists all RSA
keys for which the agent can answer a challenge. The comment
field is intended to help identify each key; it may be printed
by an application to indicate which key is being used. If the
agent is not holding any keys, howmany will be zero.
3 SSH_AGENTC_RSA_CHALLENGE
32-bit int bits
mp-int public exponent
mp-int public modulus
mp-int challenge
16 bytes session_id
32-bit int response_type
Requests RSA decryption of random challenge to authenticate the
other side. The challenge will be decrypted with the RSA
private key corresponding to the given public key.
The decrypted challenge must contain a zero in the highest (par-
tial) byte, 2 in the next byte, followed by non-zero random
bytes, a zero byte, and then the real challenge value in the
lowermost bytes. The real challenge must be 32 8-bit bytes (256
bits).
Response_type indicates the format of the response to be
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returned. Currently the only supported value is 1, which means
to compute MD5 of the real challenge plus session id, and return
the resulting 16 bytes in a SSH_AGENT_RSA_RESPONSE message.
4 SSH_AGENT_RSA_RESPONSE
16 bytes MD5 of decrypted challenge
Answers an RSA authentication challenge. The response is 16
bytes: the MD5 checksum of the 32-byte challenge.
5 SSH_AGENT_FAILURE
(no arguments)
This message is sent whenever the agent fails to answer a
request properly. For example, if the agent cannot answer a
challenge (e.g., no longer has the proper key), it can respond
with this. The agent also responds with this message if it
receives a message it does not recognize.
6 SSH_AGENT_SUCCESS
(no arguments)
This message is sent by the agent as a response to certain
requests that do not otherwise cause a message be sent.
Currently, this is only sent in response to
SSH_AGENTC_ADD_RSA_IDENTITY and SSH_AGENTC_REMOVE_RSA_IDENTITY.
7 SSH_AGENTC_ADD_RSA_IDENTITY
32-bit int bits
mp-int public modulus
mp-int public exponent
mp-int private exponent
mp-int multiplicative inverse of p mod q
mp-int p
mp-int q
string comment
Registers an RSA key with the agent. After this request, the
agent can use this RSA key to answer requests. The agent
responds with SSH_AGENT_SUCCESS or SSH_AGENT_FAILURE.
8 SSH_AGENT_REMOVE_RSA_IDENTITY
32-bit int bits
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mp-int public exponent
mp-int public modulus
Removes an RSA key from the agent. The agent will no longer
accept challenges for this key and will not list it as a sup-
ported identity. The agent responds with SSH_AGENT_SUCCESS or
SSH_AGENT_FAILURE.
If the agent receives a message that it does not understand, it
responds with SSH_AGENT_FAILURE. This permits compatible future
extensions.
It is possible that several clients have a connection open to the
authentication agent simultaneously. Each client will use a separate
connection (thus, any SSH connection can have multiple agent connec-
tions active simultaneously).
References
[DES] FIPS PUB 46-1: Data Encryption Standard. National Bureau of
Standards, January 1988. FIPS PUB 81: DES Modes of Operation.
National Bureau of Standards, December 1980. Bruce Schneier:
Applied Cryptography. John Wiley & Sons, 1994. J. Seberry and
J. Pieprzyk: Cryptography: An Introduction to Computer Secu-
rity. Prentice-Hall, 1989.
[GZIP]
The GNU GZIP program; available for anonymous ftp at
prep.ai.mit.edu. Please let me know if you know a paper
describing the algorithm.
[IDEA]
Xuejia Lai: On the Design and Security of Block Ciphers, ETH
Series in Information Processing, vol. 1, Hartung-Gorre Verlag,
Konstanz, Switzerland, 1992. Bruce Schneier: Applied Cryptogra-
phy, John Wiley & Sons, 1994. See also the following patents:
PCT/CH91/00117, EP 0 482 154 B1, US Pat. 5,214,703.
[PKCS#1]
PKCS #1: RSA Encryption Standard. Version 1.5, RSA Labora-
tories, November 1993. Available for anonymous ftp at
ftp.rsa.com.
[POSIX]
Portable Operating System Interface (POSIX) - Part 1: Applica-
tion Program Interface (API) [C language], ISO/IEC 9945-1, IEEE
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Std 1003.1, 1990.
[RFC0791]
J. Postel: Internet Protocol, RFC 791, USC/ISI, September 1981.
[RFC0793]
J. Postel: Transmission Control Protocol, RFC 793, USC/ISI, Sep-
tember 1981.
[RFC1034]
P. Mockapetris: Domain Names - Concepts and Facilities, RFC
1034, USC/ISI, November 1987.
[RFC1282]
B. Kantor: BSD Rlogin, RFC 1258, UCSD, December 1991.
[RSA] Bruce Schneier: Applied Cryptography. John Wiley & Sons, 1994.
See also R. Rivest, A. Shamir, and L. M. Adleman: Cryptographic
Communications System and Method. US Patent 4,405,829, 1983.
[X11] R. Scheifler: X Window System Protocol, X Consortium Standard,
Version 11, Release 6. Massachusetts Institute of Technology,
Laboratory of Computer Science, 1994.
Security Considerations
This protocol deals with the very issue of user authentication and
security.
First of all, as an implementation issue, the server program will
have to run as root (or equivalent) on the server machine. This is
because the server program will need be able to change to an arbi-
trary user id. The server must also be able to create a privileged
TCP/IP port.
The client program will need to run as root if any variant of .rhosts
authentication is to be used. This is because the client program
will need to create a privileged port. The client host key is also
usually stored in a file which is readable by root only. The client
needs the host key in .rhosts authentication only. Root privileges
can be dropped as soon as the privileged port has been created and
the host key has been read.
The SSH protocol offers major security advantages over existing tel-
net and rlogin protocols.
o IP spoofing is restricted to closing a connection (by
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encryption, host keys, and the special random cookie). If
encryption is not used, IP spoofing is possible for those who
can hear packets going out from the server.
o DNS spoofing is made ineffective (by host keys).
o Routing spoofing is made ineffective (by host keys).
o All data is encrypted with strong algorithms to make eavesdrop-
ping as difficult as possible. This includes encrypting any
authentication information such as passwords. The information
for decrypting session keys is destroyed every hour.
o Strong authentication methods: .rhosts combined with RSA host
authentication, and pure RSA authentication.
o X11 connections and arbitrary TCP/IP ports can be forwarded
securely.
o Man-in-the-middle attacks are deterred by using the server host
key to encrypt the session key.
o Trojan horses to catch a password by routing manipulation are
deterred by checking that the host key of the server machine
matches that stored on the client host.
The security of SSH against man-in-the-middle attacks and the secu-
rity of the new form of .rhosts authentication, as well as server
host validation, depends on the integrity of the host key and the
files containing known host keys.
The host key is normally stored in a root-readable file. If the host
key is compromised, it permits attackers to use IP, DNS and routing
spoofing as with current rlogin and rsh. It should never be any
worse than the current situation.
The files containing known host keys are not sensitive. However, if
an attacker gets to modify the known host key files, it has the same
consequences as a compromised host key, because the attacker can then
change the recorded host key.
The security improvements obtained by this protocol for X11 are of
particular significance. Previously, there has been no way to pro-
tect data communicated between an X server and a client running on a
remote machine. By creating a fake display on the server, and for-
warding all X11 requests over the secure channel, SSH can be used to
run any X11 applications securely without any cooperation with the
vendors of the X server or the application.
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Finally, the security of this program relies on the strength of the
underlying cryptographic algorithms. The RSA algorithm is used for
authentication key exchange. It is widely believed to be secure. Of
the algorithms used to encrypt the session, DES has a rather small
key these days, probably permitting governments and organized crimi-
nals to break it in very short time with specialized hardware. 3DES
is probably safe (but slower). IDEA is widely believed to be secure.
People have varying degrees of confidence in the other algorithms.
This program is not secure if used with no encryption at all.
Additional Information
Additional information (especially on the implementation and mailing
lists) is available via WWW at http://www.cs.hut.fi/ssh.
Comments should be sent to Tatu Ylonen or the SSH
Mailing List .
Author's Address
Tatu Ylonen
Helsinki University of Technology
Otakaari 1
FIN-02150 Espoo, Finland
Phone: +358-9-4354-3205
Fax: +358-9-4354-3206
EMail: ylo@ssh.fi
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